The present invention relates to a filter provided with field effect (FET) capacitors arranged for controlling their respective capacity values, each such FET capacitor having a source and a drain.
The present invention also relates to a receiver and/or transmitter provided with such a filter.
Such a filter is known from U.S. Pat. No. 4,092,619. The filter disclosed therein concerns a metal oxide semiconductor field effect (MOSFET) voltage controlled low-pass filter. The MOSFET filter comprises a plurality of easy to integrate semiconductor filtering devices coupled in series through an active buffer, which may be a source follower. The semiconductor filtering devices each have a control input or gate and form capacitors, whose capacitor values are controlled by a voltage source connected to the gates. The controllable capacitor values enable the filter cut-off frequency to be controllable too. This way a tunable, but not solely passive low-pass filter for only the audio range is realised.
Therefore it is an object of the present invention to provide a tunable, but passive filter, capable of also operating in a higher frequency range, such as the radio frequency (RF) range and capable of being integrated on chip, on only a limited IC area.
Thereto the filter according to the invention is characterised in that the source and the drain of each FET capacitor are coupled to one another.
It is an advantage of the filter according to the present invention that it is also capable of operating in the RF range and that it can in particular be used as a tunable band-pass filter in a receiver, transmitter or combined transceiver. The filter is entirely passive and therefore does virtually not consume a lot of power. Although the coupled source drain could be used as a control input for controlling the capacitor values of the respective FET capacitors, it is preferred to control the FET capacitors on their respective gates. This is because if the gate would be present in the signal path, this would result in a necessary so called Electro Static Discharge (ESD) resistor in said signal path, which would decrease a wanted high Q-factor of the filter.
An embodiment of the filter according to the invention is characterised in that each FET capacitor has a control input for voltage dependent capacity value control.
This control input will thus preferably be the gate of the FET capacitor, also because advantageously the gate current is practically virtually neglectable. A high Q-factor of the filter results, because the above mentioned ESD resistor is omitted in the signal path.
A further embodiment of the filter according to the invention is characterised in that the filter is provided with control means coupled to FET capacitor control inputs.
These control means may even advantageously be simple switching means and/or decoding means capable of switching one or more FET capacitors or FET capacitor banks into the signal path of the filter and/or capable of correspondingly controlling the capacitor values concerned respectively.
A still further embodiment of the filter according to the invention is characterised in that the FET capacitors are split in equally controlled pairs of FET capacitors.
This is particularly advantageous if the filter is built up as a symmetrically filter having a symmetrical input and a symmetrical output, because it provides a large degree of design freedom.
At wish two or more of the FET capacitors are connected in series, in order to allow larger capacitor values to be created, which in addition are variable over a large capacitor value range.
Preferably the FET capacitors are metal oxide semiconductor (MOSFET) capacitors, capable of providing small but accurately controllable capacitor values, in order to enable the filter according to the invention to be an RF filter, which is particularly suitable for operating in the radio frequency range.
The present invention also relates to a transmitter, receiver, or transceiver having an above mentioned filter, which filter is provided with field effect (FET) capacitors arranged for controlling their respective capacity values, each such FET capacitor having a source and a drain, and characterised in that the source and the drain of each FET capacitor are coupled to one another.
The above advantages also apply to the transmitters, receivers and/or transmitter/receivers concerned.